This invention relates to a process for applying a coating of pigment particles in a film forming binder by extrusion coating techniques.
Numerous techniques have been devised to form a layer of a coating composition on a substrate. One of these techniques involves the use of an extrusion die from which the coating composition is extruded onto the substrate. For fabrication of web type, flexible electrophotographic imaging members, the extrusion die must lay down very thin coatings meeting extremely precise, critical tolerances in the single or double digit micrometer ranges. Moreover, a plurality of dies may be needed to lay down up to three extruded coatings conventionally employed for flexible electrophotographic imaging members. The flexible electrophotographic imaging members may also comprise additional coatings applied by non-extrusion coating techniques so that the finished electrophotographic imaging member can contain as many as 5 different coatings. The extrusion die usually comprises spaced walls, each having a surface facing each other. These spaced walls form a narrow, elongated, passageway. Generally a coating composition is supplied by a reservoir through an inlet to a manifold that feeds the coating composition to one side of the passageway and the coating composition travels through the passageway to an exit slot on the side of the passageway opposite the reservoir. Dams are provided at opposite ends of the passageway to confine the coating composition within the passageway as the coating travels from the reservoir to the exit slot.
It has been observed that some organic pigment coating dispersions form extruded coatings that often exhibit visible defects such as brush mark streaks and wavy patterns, particularly at higher pigment concentrations.
Thus the characteristics of common extrusion systems exhibit processing deficiencies for manufacturing coated articles having precise tolerance and quality requirements.